A rod-in-tube method is one of typical methods for producing a glass preform for use in the fabrication of an optical fiber and comprises steps of inserting a glass rod constituting a core in a glass tube constituting an cladding having a lower refractive index than the core and heating and fusing the tube and the rod to collapse a gap between them and to integrate them.
The glass preform produced by the rod-in-tube method has several drawbacks such that an interface between the core and the cladding tends to have many defects (e.g., bubbles and impurities) so that the optical fiber fabricated from the glass preform has large attenuation of light transmission. To overcome the above drawback of the rod-in-tube method, it has been proposed to inject a gaseous treating agent in the gap between the tube and the rod before fusing them and then preheating the tube at a temperature of 500.degree. to 1,600.degree. C. at which the rod does not deformed (cf. Japanese Patent Publication Nos. 6261/1984 and 52935/1983). However, it has been found that when a single mode optical fiber to be used at a wavelength range longer than 1 .mu.m is fabricated from the glass preform produced by the methods disclosed in said Japanese Patent Publications, it is impossible to fabricate any optical fiber having attenuation of light transmission of less than 1 dB/km at a wavelength of longer than 1.2 .mu.m since the glass preform and in turn the optical fiber contain a large amount of hydroxyl groups.
As the result of the extensive study, it has been found that sources of the hydroxyl groups are water chemically adsorbed on the surfaces of the glass tube and rod and moisture contained in an atmosphere between the tube and the rod. Water and moisture are entrapped in the tube and/or rod during heating them to form the hydroxyl groups near the interface between them, and the formed hydroxyl groups migrate into the interiors of the core and the cladding.